[llvm] [LoopFlatten] Invalidate SCEV after flattening loops (PR #212744)

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Michael-Chen-NJU wrote:

Thanks, yes. LoopFlatten already calls SE->forgetLoop(FI.OuterLoop) after the transformation. I updated the patch to keep using `forgetLoop` instead of invalidating `ScalarEvolutionAnalysis,` and fixed SCEV so that forgetting memoized AddRec results also clears the AddRec no-wrap flags that were inferred from the old IR.

https://github.com/llvm/llvm-project/pull/212744